1. Field of the Invention
The present invention relates to image forming apparatuses, image forming methods and computer-readable storage media for forming images according to the ink-jet recording technique.
2. Description of the Related Art
The ink-jet recording technique enables a high-speed recording on the so-called plain paper without requiring a special fixing process, and the noise generated during the recording is negligibly low. Hence, the ink-jet recording technique is suited for office use. In addition, various systems employing the ink-jet recording technique have been proposed and reduced to practice. The ink-jet recording technique typically uses an ink chamber and a recording head having a nozzle communicating with the ink chamber. By applying pressure on the ink within the ink chamber depending on image information, ink drops are jetted from the nozzle onto a recording medium such as paper and film, to form an image of the image information on the recording medium.
An image forming apparatus employing the ink-jet recording technique, such as an ink-jet printer, can record images on various kinds of recording media because the image is formed by jetting the ink from the recording head onto the recording medium without requiring the recording head to make contact with the recording medium. The ink-jet printers in general can be categorized into a serial type and a line type.
The serial type ink-jet printer forms the image by causing a recording head to reciprocate in a main scan direction which is perpendicular to a sub scan direction or a medium transport direction in which the recording medium is transported. On the other hand, the line type ink-jet printer has recording heads that are fixedly arranged along approximately the entire width of the recording medium. However, both the serial type ink-jet printer and the line type ink-jet printer have a common problem to be solved, namely, lines, unevenness or blur that may appear in the image that is formed on the recording medium.
Because the serial type ink-jet printer forms the image on the recording medium while transporting the recording medium, lines, unevenness or blur may be generated in the image at a joint of the scans due to causes such as an error in the transport of the recording medium and a tilt in the recording head. In addition, the serial ink-jet printer may have an elongated recording head structure in which a plurality of recording heads are connected in order to increase the printing speed. In the serial ink-jet printer having the elongated recording head structure, the lines, unevenness or blur may be generated in the image at a joint of the recording heads due to a positional error caused by a mounting or assembling error. The positional error at the joint of the recording heads is fixedly determined by the precision with which the recording heads are mounted or assembled, and the lines, unevenness or blur generated in the image cause a serious problem because it is difficult to adjust and minimize the positional error after the recording heads are mounted or assembled.
The line type ink-jet printer has the recording heads that are connected and fixedly arranged along approximately the entire width of the recording medium. Because the recording heads are fixed, the image formation on the recording medium is completed in one head scan, and the so-called 1-pass (or 1-scan) recording is performed. For this reason, the lines, unevenness or blur generated in the image cause an even more serious problem in the case of the line type ink-jet printer since the lines, unevenness or blur generated in the image cannot be reduced by a measure such as performing the head scan a plurality of times.
The lines, unevenness or blur generated in the image at the joint of the scans and the lines, unevenness or blur generated in the image at the joint of the recording heads are basically the same. For this reason, the lines, unevenness or blur generated in the image at the joint of the scans and at the joint of the recording heads will hereinafter simply referred to as the lines, unevenness or blur generated in the image at the “joint”. There is a proposed technique that reduces the lines, unevenness or blur generated in the image at the joint by overlapping the nozzles. More particularly, this proposed technique overlaps the nozzles at ends of the recording head structure and forms overlapping dots (or pixels) by a plurality of recording heads, in order to reduce the lines, unevenness or blur generated in the image by increasing or decreasing the dot recording density.
For example, a Japanese Laid-Open Patent Publication No. 2006-240043 proposes a method of determining the nozzles from which the ink is to be jetted in an overlapping region of the nozzles, based on a random number.
According to the proposed method described above, it is possible to reduce the lines, unevenness or blur generated in the image within the overlapping region. However, because the nozzles from which the ink is to be jetted in the overlapping region is determined based on the random number, a state where the dots are continuously formed by the nozzles and a state where the dots are not continuously formed by the nozzles coexist. For this reason, a driving frequency of a signal driving a driving part of the nozzles in a non-overlapping region where the dots are continuously formed becomes different from that in the overlapping region where the dots are not continuously formed. For example, in the case of a solid printing that prints the dots at all pixel positions of the image to be printed, the driving frequency may be 20 kHz in the non-overlapping region where the dots are continuously formed, while the driving frequency may be 10 kHz in the overlapping region where the dots are not continuously formed.
But the state of the ink surface within the recording head at each of the nozzles, that is, the meniscus surface of the nozzles, is affected by the driving frequency, and the amount of ink drops jetted from the nozzles and the landing positions of the ink drops on the recording medium deviate depending on the driving frequency due to the vibration state of the ink surface that differs depending on the driving frequency. In this specification, such deviations in the amount of ink drops jetted and the landing positions of the ink drops will be referred to as “deviations in the ink drops”. Furthermore, characteristics of the recording head having the meniscus surface of the nozzles affected by the driving frequency, that cause the deviations in the ink drops, will be referred to as “frequency characteristics of the recording head”. Consequently, a banding may occur in the overlapping region due to the deviations in the ink drops jetted in the overlapping region caused by the driving frequency which differs between the non-overlapping region and the overlapping region, that is, caused by the frequency characteristics of the recording head.